I've had a fair share of nightmarish Gym Leaders but the one who takes the cake for me is definitely Clair in HG/SS.

That Kingdra of hers only has a single weakness to Dragon, pretty darn good stats and moves that are able to massacre your whole team due to a combination of high base power and STAB. Also, there is a highly likely chance your team would be lacking a Dragon-type attack at this point meaning you have to just whittle away at her team's health unless if you land a lucky critical hit on her.

Trust me, she always makes me shiver, and I hate battling strong Dragon-types in the game.